Plastic components, having a wide variety of applications, are produced from modern, state of the art, engineering grade molding resins. Typical of these resins used in a molding process are polyphthalamide available under the trademark AMODEL from AMOCO, Inc., of Chicago, Ill.; polycyclohexylenedimethyleneterephthalate available under the trademark EKTAR from Eastman Kodak, Inc. of Rochester, N.Y.; polyphenylene sulfide available under the trademark RYTON from Phillips Petroleum, Inc. of Bartlesville, Okla.; and liquid crystal polymer available under the trademark VECTRA from Celanese, Inc. of Wilmington, Del.
Molding conditions for plastic molded component parts from these and similar resins are subject to many variable and stringent quality constraints or requirements not usually common to plastic components produced from standard commodity grade molding resins. Among those requirements for the molded products are (1) very high strengths, (2) resistance to high ambient heat, (3) exceptional co-planarity requirements especially in the electronic components field, (4) exceptionally tight "as molded" tolerances and (5) greatly reduced dimensional variability around a specified standard in the as molded state.
In order to achieve these stringent goals, and to meet the mandated requirements of today's sophisticated injection molded component customer; the plastics industry has produced increasingly complex and difficult to process molding resins. The current industry trends indicate that there will not be a lessening, but instead, an ever increasing list of physical demands placed on injection molded parts Consequently, tremendous increases in both the complexity and tightened processing parameters of engineering grade molding resins are virtually assured.
These critical requirements are complicated by a number of additional factors. Ever higher loadings of reinforcing materials such as; glass fibers, ARAMID (ARAMID being a registered trademark of AMOCO, Inc. of Chicago, Ill.) fibers, carbon fibers, ceramics, magnetic fillers, mineral fillers, graphite fillers, and other fillers are commonly used in these parts. The added fillers reduce the viscosity of the melted resin, which is to be molded.
Each of these resins has an ideal set of conditions for processing or molding the resin. Reducing or tightening of the ideal processing window is caused by a number of factors including, but not limited to, process temperatures, processing pressures, heat degradation of the resin melt, and susceptibility to increased processing shear damage for highly engineered polymers. These resins are also desired to have a higher flex modulus or a higher notched Izod impact strength.
Other processing difficulties associated with today's increasingly complex and changeable engineering grade polymers are increased mold and tool wear and deterioration due to the abrasiveness of fillers. Higher concentrations of volatile and corrosive chemicals also result in accelerated corrosion and wear of mold components. These additives result in reduced ability to adequately, and consistently, fill individual mold cavities.
Higher injection pressures and faster resin melt injection rates do not compensate for these problems. These factors result in increased opportunity for damage to occur in ever smaller and more delicate, (especially in electronics applications), injection mold components resulting from highly reinforced engineering grade polymers. Increased importance of optimized resin melt delivery at a sufficient temperature to the respective mold cavity or cavities thus becomes clear.
In terms of processing consistency, today's plastic processor must be constantly aware of, and strive to improve upon, all variable and controllable segments of the plastic injection mold, and molding process. The processor must provide for optimized part quality by monitoring, and, where possible, controlling molding resin melt characteristics and conditions such as distance of melt travel in the mold, distance of melted resin travel in the cavity, directional control of resin melt stream, configuration of the part being molded, resin viscosity and reinforcement materials.
Heat is another important factor in molding. The first heat factor playing a part in molding relates to the melt temperature of the resin. Secondly, process induced heat from shear conditions produced by the molding machine and the mold configuration itself adds to the heat inherent in such a process. These heat factors must be carefully coordinated to avoid adverse effects on the molded resin, and the resultant deleterious effect on the molded product.
With the appropriate process and tooling controls, it is possible to eliminate, or control as much as possible, the location of flow or melt knit lines. By monitoring and controlling the above parameters, the molder is able to optimize part strength and part consistency, while both minimizing part warpage and tool wear and maximizing overall tool life. Such monitoring and controlling is highly difficult to accomplish in view of the other molding requirements.
One of the control methods employed by the molder, when economically feasible and warranted by high production volumes, is centralized resin melt delivery to the injection mold cavity. Centralized resin melt delivery is especially important in the case of high temperature, reinforced, engineering grade molding resins. These types of resins easily deteriorate under shear induced process heats. Also the reinforcement fibers contained therein are subject to damage when optimum flow characteristics are not achieved or are overlooked by the molder.
Two of the tool construction methods used to accomplish central resin melt delivery to the cavity of the injection mold are commonly known as a "hot runner" or "runnerless" type molds, and three-plate molds. While the above types of injection molds are able to accomplish the task of central resin melt delivery to the mold itself, they normally rely on a conventional gating approach to accomplish the dispersion of resin melt within the cavity itself. This molding process typically results in a less than optimum delivery of the resin melt within the most important section of the injection mold, the mold cavity itself.
These problems become especially clear in a molded component with a hollow, or cored through configuration, such as is found in a common electronic component called a pin grid array. This is an especially problematic condition. It is problematic due to induced stress and shear caused by the melt flow passing around the standing core section which forms the hole itself, as well as the multiplicity of small and delicate core pins found within the mold cavity.
One method of dispersing the resin melt within the cavity in a centralized and relatively consistent manner throughout the mold cavity is by use of the fan gate or diaphragm type gate method. This type of gating, while providing an acceptably uniform dispersion of the resin melt within the cavity itself, requires expensive, time consuming trimming or die cutting of each individual molded plastic component. Additionally, construction and ongoing maintenance of costly trim fixtures or dies is required.
Thus, the molding of a suitable resin product causes a number of problems. The solution to one problem can and does create an additional problem or exacerbate another problem. The main objective is to minimize all problems, while maximizing product quality in the as-molded state, while at the same time, substantially minimizing or eliminating the post molding treatment.